Method and system for e-mail message transmission

ABSTRACT

An e-mail firewall applies policies to e-mail messages transmitted between a first site and a plurality of second sites. The e-mail firewall includes a plurality of mail transfer relay modules for transferring e-mail messages between the first site and one of the second sites. Policy managers are used to enforce and administer selectable policies. The policies are used to determine security procedures for the transmission and reception of e-mail messages. The e-mail firewall employs signature verification processes to verify signatures in received encrypted e-mail messages. The e-mail firewall is further adapted to employ external servers for verifying signatures. External servers are also used to retrieve data that is employed to encrypt and decrypt e-mail messages received and transmitted by the e-mail firewall, respectively.

RELATED APPLICATIONS

This application is a continuation of, and claims priority to, U.S.patent application Ser. No. 09/887,313 filed Jun. 22, 2001, Method andSystem for E-Mail Message Transmission, now U.S. Pat. No. 7,127,741.

In addition, this application is related to U.S. patent application Ser.No. 09/180,377, E-Mail Firewall With Stored Key Encryption/Decryption,filed on Jul. 23, 1998, which was the National Stage of InternationalApplication PCT/US98/15552 and issued as U.S. Pat. No. 6,609,196, andwhich claims priority to U.S. Provisional Patent Application 60/053,668filed on Jul. 24, 1997. Application Nos. 09/887,313, 09/180,377 and60/053,668 are each incorporated herein by reference.

TECHNICAL FIELD

This application pertains generally to the field of computer securityand more specifically to security for electronic mail systems.

BACKGROUND ART

The widespread use of electronic mail (e-mail) and groupwareapplications coupled with the growth and ubiquity of the Internet haveopened new avenues for business level communications and electroniccommerce. Organizations are increasingly relying on e-mail for thetransfer of critical files such as purchase orders, sales forecasts,financial information, and contracts, both within the organization and,increasingly, with other organizations via the Internet. In thissetting, these files are now tangible information assets that must beprotected.

A number of conventional security measures exist to ensure theconfidentiality and integrity of modern data communications. Forexample, traditional firewalls prevent network access by unauthorizedusers. Secure sockets technology allows for data to be passed securelyover the World Wide Web (WWW). E-mail, however, which is by far the mostprominent application over the Internet, still remains problematic, froma security standpoint, for most organizations. Many traditionalfirewalls simply limit access to information protected by the firewallbut do not contain the capability to limit transfer of information, intoor out of an organization, by way of e-mail. This can lead toinadvertent or deliberate disclosure of confidential information frome-mail originating within an organization and introduction of virusesfrom e-mail entering an organization.

One solution to protecting confidentiality of e-mail messages is byencrypting such messages. Further security is available by way ofdigital signatures, which provide for authentication of e-mail messagessenders. Encryption and authentication are both supported in the S/MIME(Secure/Multipurpose Internet Mail Extensions) messaging protocoldefined in documents generated by the Internet Engineering Task Force(IETF) entitled “S/MIME Message Specification” (1997) and “S/MIMECertificate Handling” (1997). Individual users can encrypt/decrypt andauthenticate e-mail messages using commercially available software.However, the configuration, installation and use of software to performsuch tasks is often complex and therefore can detract from the inherentease of use of e-mail as a means of communication. Moreover, anorganization wishing to use such software must rely on individual usersto encrypt all necessary messages without means of any centralizedcontrol. In addition, many conventional firewalls contain no capabilityto control the content or format of certain messages that enter or exitan organization. For example, many conventional firewalls contain nocapability to ensure that e-mail meeting certain criteria such ascontent or source and/or destination address or domains, is encrypted.In addition, many conventional firewalls contain no capability tocontrol unwanted messages entering an organization such as unsolicitede-mail advertising.

There is accordingly a need for an e-mail firewall that providesimproved centralized control over e-mail messages exiting and enteringan organization.

SUMMARY OF THE INVENTION

In a principal aspect, the present invention provides an e-mail firewallfor screening e-mail messages originating in, or entering into acomputer network. Embodiments employing the principles of the presentinvention advantageously take the form of an e-mail control system thatcontrols e-mail messages transmitted from and received by a computingsite. The e-mail control system includes a message encryptor, whichencrypts, in accordance with at least a first stored encryption key, afirst designated type of message transmitted from the computing site. Amessage decryptor decrypts, in accordance with at least a second storedencryption key, a second designated type of message, which is receivedby the computing site. A filter monitors messages, after decryption bythe decryptor and before encryption by the encryptor, in accordance withchangeable filter information.

In one embodiment, the invention provides an e-mail firewall, whichcooperates with a remote publicly accessible security server to securelytransmit e-mail messages. The system includes a message encryptor, whichencrypts an e-mail message in accordance with at least one encryptionkey. The system further includes a lookup module, which queries theremote security server for an encryption key (including relatedencryption data), associated with at least one target server for thee-mail message. Finally, the system includes a transmission module,which transmits the e-mail message to at least one target server, forwhich encryption data was retrieved by the lookup module. Optionally,the system includes a signature lookup module to retrieve signaturesassociated with the e-mail message source (sender or system). Thesignature is then applied to the e-mail message to allow for therecipient to authenticate the message source.

In another embodiment, the invention facilitates an e-mail messagetransmission method. The method receives an e-mail message into atransmission server. The e-mail message is associated with at least onerecipient server, which is coupled to the transmission server by anetwork connection. The method retrieves encryption data correspondingto at least the recipient server by accessing a lookup server, which iscoupled to the transmission server by a network connection. The methodthen encrypts the e-mail message in accordance with the retrievedencryption data. Finally, the method transmits the encrypted e-mailmessage to the recipient server.

In yet another embodiment, the invention provides an e-mail messagereception method. The method receives an encrypted e-mail message from aremote server. The method decrypts the e-mail message in accordance withencryption data. The method then extracts digital signature data fromthe e-mail message. Next, the method verifies the extracted signature byaccessing a signature verification server. Finally, the method processesthe e-mail message in accordance with the results of the verifying step.Alternatively, the method employs a local repository of signatures toverify signed e-mail messages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 of the drawings is a block diagram showing a plurality of e-mailnetworks which are coupled by way of the Internet, and which employ ane-mail firewall employing the principles of the present invention;

FIG. 2 of the drawings is a block diagram of a preferred embodiment ofan e-mail firewall;

FIGS. 3 and 4 are block diagrams illustrating further details ofoperation of the e-mail firewall of FIG. 2;

FIGS. 5( a), 5(b) and 5(c) are block diagrams illustrating alternativesecure e-mail communication mechanisms;

FIGS. 6( a) and 6(b) are flowcharts illustrating operation of apreferred embodiment of an e-mail firewall;

FIG. 7 is a block diagram showing further details of a portion of FIGS.6( a) and 6(b);

FIG. 8 is a flowchart illustrating a signature verification operation;

FIG. 9 is a flowchart illustrating a signature insertion operation;

FIG. 10 is a flowchart illustrating certificate lists generation;

FIG. 11 is a flowchart illustrating applying encryption to e-mailmessage transmission; and

FIG. 12 is block diagram showing an arrangement of email firewalls andan external certificate lookup server.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 of the drawings, e-mail networks 101 and 102 are coupled toe-mail network 103 by way of a Wide Area Network (WAN) 104 such as theInternet. Disposed between the internet 104 and e-mail network 101 and103 are an access firewall 106 and an e-mail firewall 105. E-mailnetwork 102 is coupled to Internet 104 only by access firewall 106.1.E-mail networks 101, 102, and 103 may each take a conventional form. Forexample, e-mail networks 101-103 may take the form of a Local AreaNetwork (LAN) or a plurality of LANs which support one or moreconventional e-mail messaging protocols. Access firewalls 106 may alsotake a conventional form. Access firewalls 106 operate to limit accessto files stored within a computer network, such as e-mail networks101-103, from remotely located machines. E-mail firewalls 105(individually shown as 105.1 and 105.2) advantageously take a form asdescribed in further detail herein to control transmission of electronicmail messages between an internal site and one or more external sites.An internal site for e-mail firewall 105.2, by way of example, may takethe form of e-mail network 103. External sites for e-mail firewall 105.2are any sites not contained in e-mail network 103. For example, externalsites for e-mail firewall 105.2 are any sites in e-mail networks 101 and102 as well as any other sites coupled to Internet 104. E-mail firewall105 is preferably positioned on the “safe-side” of the access firewall106. FIG. 1 should be understood as showing, by way of an example, theprinciples of the embodiments described herein. The access firewalls 106are shown only for purposes of explanation and are not required foroperation of embodiments employing the principles of the presentinvention.

Preferably the e-mail firewall 105 takes the form of a program executingon a conventional general purpose computer. In an exemplary embodiment,the computer executes the Windows NT or Windows 2000 operating systemsavailable from Microsoft Corp., of Redmond, Wash. In other embodiments,the computer executes a Unix operating system such as Solaris from SunMicrosystems, of Mountain View, Calif. Although e-mail firewall 105 isshown in FIG. 1 as operating on e-mail messages between an internal siteand an external site, the e-mail firewall 105 may also be used toexchange messages between two internal sites for computer networks withSMTP compliant messaging backbones.

FIG. 2 of the drawings illustrates in block diagram form the majorfunctional components of e-mail firewalls 105.1 and 105.2. In FIG. 2, aSimple Mail Transfer Protocol (SMTP) relay module 202 performs thefunctions of a conventional SMTP relay host. An example of an Internetrelay host is a UNIX Send mail program. The SMTP relay module 202transmits and receives e-mail messages such as shown at 204 to and froman internal site 210 and external sites 212. E-mail message 204 takesthe form of a conventional e-mail message which contains a plurality ofuser specified information fields, such as source field 205 specifyingan e-mail address for the source of the message 204, a destination field206 specifying one or more destination e-mail addresses for the message204, a subject field 207 specifying a subject for the message 204, abody field 208 specifying the body of the message 204 containing textualand/or graphics data, and an optional attachment field 209, specifyingone or more files to be transmitted with the message 204. Other userspecified fields include, but are not limited to, priority of themessage, identity of the sending agent, and the date and time of themessage.

E-mail message 204 may be encoded in accordance with one of a pluralityof encoding formats as explained in further detail below. SMTP relaymodule 202 preferably takes a conventional form of a software modulewhich receives and transmits e-mail messages in accordance with theSimple Mail Transfer Protocol as specified by ‘Internet RFC 821.’ TheSMTP protocol is not critical to the invention. In other embodiments,the SMTP relay module is replaced with a module that receives and/ortransmits messages in other formats such as the File Transfer Protocol(FTP), the Hyper-Text Transfer Protocol (HTTP), the Network NewsTransfer Protocol (NNTP), or the Internet Relay Chart (IRC).

In one embodiment, the SMTP relay module 202 is configured to use theDomain Name System (DNS) to determine routing to message recipients oralternatively is configured to relay messages to at least oneadministrator specified SMTP host. If DNS is selected, at least one SMTPhost is specified to allow for default message forwarding even if DNSservice is not available. The routing option can be overridden on aper-domain basis. The SMTP relay module 202 advantageously allowsinbound and outbound SMTP connections to be limited from or to specifichosts and allows connections to or from specific SMTP hosts to bedenied. Preferably, the SMTP relay module 202 transmits messages thatinclude text messages and binary data e-mail messages, as is known inthe art. The following illustration refers to a generic routing server,which facilitates some of the functionality provided by the SMTP relaymodule 202 to transmit e-mail messages in accordance with the invention.

As discussed above, the SMTP relay module 202 receives data identifyingintended recipients for a subject e-mail message. Preferably, the dataincludes e-mail addresses for the intended recipients. The SMTP relaymodule 202 receives data identifying an e-mail message for the intendedrecipients. Example e-mail messages include combinations of, orindividual, text messages, graphical image data, audio data, video data,meta data, database records, binary data, executables, and compressedarchives.

In another embodiment, the SMTP relay module 202 also receives deliveryparameters, such as message priority, and other optional parameters forthe e-mail message. In one embodiment, a security preference specifiesthat servers cooperating in the delivery of the e-mail message shouldemploy secure transmission protocols. The SMTP relay module 202preferably stores the e-mail message in a temporary location beforetransmission. In one embodiment, the e-mail message is routed separatelyto each intended recipient. In some embodiments, routing optimizationtakes place if the routing server detects that two or more recipientsare associated with a common server. Accordingly, a single copy of thee-mail message is routed to the recipient's server, indicating that thee-mail message is intended for multiple recipients.

FIG. 3 illustrates the manner in which messages received by the SMTPrelay module 202 from internal site 210 and external site 212 areprocessed by policy engine 214. Policy engine 214 accepts messages fromSMTP relay module 202 and determines which policies are applicable to amessage by building a list 302 of sender policies for the sender(source) 204 of the message, and building a list 302, 306, and 308 ofrecipient policies for each recipient. The policy engine 214 then callsthe policy managers 216 to apply each policy. The different types ofpolicies have a predetermined priority in which they are applied. Forexample, decryption policies are applied before other policies, to allowthe policies that operate on the body 208 of the message to be able toaccess the contents contained therein. In an alternative embodiment, theorder in which the policies are applied is selectable by a systemadministrator. Access manager policies get applied after decryptionpolicies and then the other policy managers are called repeatedly in theorder implied by the policies to be applied to the message. The policyengine 214 then receives results from policy managers 216 and transmitsmessages to SMTP relay module 202 in accordance with the receivedresults. The results received by the policy engine 214 comprise actionssuch as disposition, annotation, and notification described in furtherdetail herein. The result of processing of a message 204 by policyengine 214 can result in generation of a plurality of additionalmessages, for example, for notification to the sender or recipient, orto the system administrator. In a preferred embodiment, the policyengine 214 is implemented as a program executed by a digital computer.

Policy managers 216 operate to enforce policies entered by anadministrator of e-mail firewall 105. Policy managers 216 preferablycomprise a plurality of modules for enforcing administrator configuredpolicies, directed to specific aspects of e-mail messages. For example,in e-mail firewall 105, policy manager 216 implements a plurality ofmanager modules including an access manager 218, a content manager 220,a format manager 222, a virus manager 224, and a security manager 226.Policy managers 216 are preferably developed by inputs entered by anadministrator by way of configuration module 230. Configuration module230 also operates, in response to information entered by anadministrator, to configure SMTP relay 202 and policy engine 214. Thepolicy managers shown in FIG. 2 and described herein are merelyillustrative of an exemplary embodiment. Other types of policy managersare contemplated as being within the principals described herein.

Access manager 218 provides enforcement of access control policies suchas destinations to which e-mail is prohibited from being sent, orsources from which e-mail cannot be received. Access manager 218 canalso filter messages that exceed a maximum message size determined by anadministrator, or which contain specific words in the subject field 207of the message. Access manager 218 can also filter a message by thepriority of the message specified by the user. For example, highpriority messages can be passed through immediately, while low prioritymessages are stored in a queue (explained in further detail inconnection with FIG. 7). Access manager 218 can also filter messages bythe date and/or time of transmission of the message. For example,messages transmitted between certain hours of the day or on certaindays, such as weekends or holidays may be retained or further filteredby, for example, content manager 220.

Content manager 220 supports the enforcement of content controlpolicies. Preferably content manager 214 supports filtering by one ormore of the following criteria: (a) specific words, or word patterns, inthe body 208; (b) specific words in the subject 207; (c) attachment 209(all or by name/type); (d) specific words, or word patterns, in theattachment 209. Content control policies, and other appropriatepolicies, can also be specified to require certain material, such as forexample, certain notices or disclaimers. Virus manager 224 supports theenforcement of virus control policies by detecting virus infected e-mailattachments. Virus manager 224 preferably detects viruses contained in aplurality of compressed file formats including PKZip, PKLite, ARJ,LZExe, LHA, and MSCompress. Virus manager 224, by way of example, mayuse a commercially available virus scanning engine. Virus manager 224also preferably applies policies on “clean messages,” that is, messagesthat have been scanned for a virus and found to be free of any viruses.In this embodiment, a “clean stamp” annotation is added to suchmessages, indicating that no viruses were detected.

Format manager 222 provides conversion of an e-mail message from a firstformat to a second format. In a preferred embodiment, format manager 222converts messages from conventional UUENCODE format to MIME format.Preferably format manager 222 converts messages prior to messageprocessing by other policy managers.

Security manager 226 preferably enforces a plurality of e-mailencryption policies. Preferably, security manager 226 enforces a clientsecurity usage policy, a preserve encryption policy, a plain text accesspolicy, and default action policies. Security manager 226 also applieson behalf of users proxy encryption and signature policies, as discussedin further detail in connection with FIG. 5( b).

In one embodiment, client security usage policies specify that certainusers, under certain conditions, should perform encryption or signature,or both, at the desktop. Additional criteria can be set to indicate whenthis policy should be enforced. For example, an e-mail from a company'sCEO to the company's legal counsel by the domain or full e-mail addresscan be specified to require either encryption, signature, or both, toenforce attorney-client privilege and to preserve encryption policies.Moreover, client security usage policies can be used to specify thatmessages, which are already in encrypted form and perhaps meet someother criteria, should be preserved. Thus, such messages are notprocessed, modified, or encrypted by the e-mail firewall 105.Furthermore, the security policy may also select varying encryptionmethods as a result of applying policy to transmitted e-mail. Plain textaccess policies require that the e-mail firewall 105 is designated as arecipient on certain types of specified messages. The e-mail firewall105 is designated as a recipient on encrypted messages in order to applyaccess, content, virus, and other policies on the message. Plain textaccess policies can also be used to send a signed notification to thesender of a message as a way of providing the sender with the e-mailfirewall's 105 public key. Default action policies indicate the actionto be taken on messages, which are not encrypted and will not beencrypted by the e-mail firewall 105, and which might meet some othercriteria. The default action policy type is used to ensure that certainmessages get encrypted somewhere, whether at the desktop or by thee-mail firewall 105.

Policies are preferably entered by an authorized administrator by way ofconfiguration module 230 which preferably takes the form of a programexecuting on a stored program computer. Policies can advantageously beapplied to users, either individually or by e-mail domains or othergroupings. FIG. 4 shows an example of how policies are applied. Userscan be organized in a hierarchical directory-type structure tofacilitate grouping of users and/or domains. If a policy is applied to agiven directory then sub-directories corresponding to the givendirectory inherit such policies. For example, in FIG. 4, policy 1applies to sub-directory 404 and thus applies to all sub-directories,domains and users, such as sub-directory 412, user 408, and domain 410,corresponding to sub-directory 404, unless that policy is explicitlyoverridden by another policy applied to a particular sub-directory or toan intervening sub-directory. For example, policy 3 will override policy1, for users shown at 408, where there are conflicts between policy 1and policy 3, and will supplement policy 1, where there are noconflicts. Exception 1 will override policies 1 and 3 for the particularexception specified in exception 1. As further shown in FIG. 4, policy 1applies to users 414, 416, and 418, and is overridden by policy 2 forusers 414, 416, and 418 in the event of conflicts, and is supplementedwhere there are no conflicts. This advantageously allows policies to beeasily applied to groups of users. The exact manner in which thepolicies are stored is not critical and a variety of means and formatsof storage may be employed.

E-mail messages 204 received and/or transmitted by SMTP relay 202 arepreferably encoded in accordance with the S/MIME (Secure/MultipurposeInternet Mail Extension) protocol, as specified by the InternetEngineering Task Force in documents entitled “S/MIME MessageSpecification” (1997) and “S/MIME Certificate Handling” (1997).Advantageously, the S/MIME protocol builds security on top of theindustry standard MIME protocol according to Public Key CryptographyStandards (PKCS) specified by RSA Data Security, Inc. S/MIMEadvantageously offers security services for authentication using digitalcertificates, and privacy, using encryption. Digital certificates arepreferably implemented in accordance with the X.509 format as specifiedin “Information Technology—Open Systems Interconnection—The Directory:Authentication Framework,” also known as “ITU-T Recommendation X.509”(June 1997). Encryption is preferably performed by one of the followingsymmetric encryption algorithms: DES, Triple-DES, RC2, and otheralgorithms introduced by revisions of the S/MIME standard. The S/MIMEprotocol is well known and widely used and provides encryption anddigital signatures and is therefore preferable as a communicationsprotocol. The precise details by which the protocol operates is notcritical. Moreover, it should be understood that other secure messagingprotocols such as PGP (Pretty Good Privacy) or Open PGP, as specified bythe ITF working group, may also be used.

Access manager 218 is the first policy manager to process e-mail message204. Access manager 218 operates only on message header informationwhich is not encrypted. Thus, access manager 218 may operate on ane-mail message 204 prior to decryption by S/MIME engine 215. The term“message header information” generally refers to portions of messageexcluding the body 208 (and commonly referred to as message text), andattachments 209. Thus, the header information includes the source,destination and subject fields (205, 206, 207). Optional header fieldsinclude date/time stamp, priority, and sending agent. The remainder ofthe modules operate on the message 204 after processing by S/MIME engine215. As previously noted, format manager 222 preferably operates onmessages prior to operation by other managers such as virus manager 224,security manager 226, and content manager 220.

The S/MIME protocol allows two sites which support the S/MIME protocolto exchange secure e-mail messages 204. A type of virtual privatenetwork (VPN), as shown in FIG. 5( a), can be achieved if both thetransmitting and receiving site perform S/MIME functions. The resultingVPN, termed herein an “object level e-mail VPN,” providesencryption/signature and/or decryption/verification of messages betweentransmitting and receiving site(s). In the object level e-mail VPN shownin FIG. 5( a), each object (message) is encrypted individually and sentover a standard (SMTP) transport medium, where each object (message) isdecrypted at the other end. Advantageously, the object level e-mail VPNdoes not require a secure real-time connection as required byconventional VPNs. As shown in FIG. 5( a), mail servers 105.1 and 105.2perform functions described herein for e-mail firewall 105, and as aresult, achieve an object level e-mail VPN between them. E-mail that isencrypted and transmitted between servers 105.1 and 105.2 is protectedfrom disclosure to third parties, despite the fact that e-mailtransmitted via the Internet 104 may pass through numerous unsecuredservers before reaching its destination. Accordingly, one may appreciatethat it is not required for the intermediate e-mail relay serversbetween servers 105.1 and 105.2 to support encryption or decryption ofmessages.

In one embodiment, in such an exchange, e-mail firewalls 105.1 and 105.2provide key pair and public key certificate generation and provideautomated or manual public key certificate exchange with the otherS/MIME server. In addition, e-mail firewalls 105.1 and 105.2 allow:identification of the other S/MIME server through directory domainrecords, association of directory domain records with servercertificates and selection of encryption/signature algorithms and keylengths. The directory domain records, and the directory user recordsreferred to below, are as described in FIG. 4.

Exchange of S/MIME encoded messages may also be performed between thee-mail firewalls 105.1, 105.2 and an S/MIME client coupled in a serverthat does not perform S/MIME functions. FIG. 5( b) illustrates anexchange between e-mail firewall 105 and a S/MIME client coupled to anon-S/MIME server 506. In FIG. 5( b), server 105.1 encrypts and decryptsmessages on behalf of client 502.2 and generally provides the functionsdescribed above for e-mail firewalls 105.1 and 105.2. Specifically, insuch an exchange, e-mail firewall 105.1 provides key pair and public keycertificate generation and provides automated or manual public keycertificate exchange with the client 508.1. In addition, e-mail firewall105.1 allows: identification of the client 508.1 through directory userrecords, association of directory user records with user certificatesand selection of encryption/signature algorithms and key lengths. Client508.1 provides encryption/decryption services to allow messages to betransmitted securely through server 506 by supportingencryption/decryption services. A specific type of object level VPN,referred to herein as “proxy security,” is achieved in FIG. 5( b)between the server 105.1 and the client 508.1. In proxy security, atleast one client is involved in performing encryption/decryption, suchas client 508.1 in FIG. 5( b). This is in contrast to the arrangement ofFIG. 5( a), where the encryption/decryption services performed byservers 105.1 and 105.2 is transparent to the clients 502.1 and 502.2.

In FIG. 5( a), communications between servers 105.1 and 105.2 aresecure, but communications between clients 502.1 and 502.2 and theirrespective servers 105.1 and 105.2 are not necessarily secure. In manysuch installations, security is not necessary because the client 502.1and the server 105.1 typically communicate over a common LAN, which isprotected from the Internet by a standard firewall. However, if suchsecurity is desired, the clients 508.1 and 508.2 can also be equippedwith encryption/decryption services to perform proxy security, as isshown in FIG. 5( c). The servers 105.1 and 105.2 perform the samefunction described above in connection with FIG. 5( a) and thereforeachieve an object level VPN. In addition, the clients 508.2 and 508.1allow secure communications with the corresponding servers 105.1 and105.2. It should be noted that the encryption/decryption performed byservers 105.1 and 105.2 can be independent of the encryption performedby the corresponding clients 508.2 and 508.1. For example, a message byclient 508.2 to client 508.1 may be encrypted when transmitted to server105.1, decrypted by server 105.1 and subjected to appropriate actions bythe policy managers. The message may then be encrypted for transmissionto server 105.2, decrypted by server 105.2 and subjected to appropriateactions by the policy managers, and encrypted for transmission to client508.1 which decrypts the message. Alternatively, a message by client508.2 to client 508.1 may be encrypted by client 508.2, be subjected toappropriate actions to non-encrypted portions, such as the destinationfield, and then the entire message, including the portions not encryptedby client 508.2, can be encrypted again by server 105.1 for transmissionto server 105.2, which decrypts the encryption by server 105.1, andtransmits the message to client 508.1 for decryption of the encryptionperformed by client 508.2. Several combinations of the foregoing twoscenarios are possible. In another embodiment, the client to serverconnection is protected by means other than object level security suchby using a Secure Socket Layer (SSL) connection while the connectionbetween servers is by an object level VPN in accordance with theinvention.

Each e-mail message 204 processed by e-mail firewall 105 is processed inaccordance with the steps shown in FIGS. 6( a) and 6(b). FIG. 6( a) is aflowchart showing operation of the e-mail firewall 105 in response to areceived message. FIG. 6( b) is a flowchart showing operation of thee-mail firewall 105 prior to transmitting a message. The messagesprocessed by e-mail firewall 105 may be received from an internal sitefor transmission to an internal site, or may be received from aninternal site for transmission to an external site, or may be receivedfrom an external site for transmission to an internal site. Any singlemessage may include internal and external destinations 206. The stepsshown in FIGS. 6( a) and 6(b) are preferably performed by generation ofsender and recipient policies shown in FIG. 3. For multipledestinations, the steps shown in FIG. 6( b) may therefore be performeddifferently and have different results for different destinations.

Turning to FIG. 6( a), at 602, the e-mail firewall 105 determines ifdecryption of portions of the message 204 is required. If so, then at604, decryption is performed in accordance with stored private keys 628.Storing private keys is well known in the art of public keycryptography. After decryption, or if no decryption is required, thee-mail firewall 105 applies policy managers 216, which can perform fourtypes of actions (shown at 610, 612, 614, 616, and 620) on e-mailmessage 204 for each policy. Criteria actions 610 present filteringcriteria selected by the administrator. Exception actions 612 determinewhich criteria 610 are excluded. Multiple criteria 610 can be selectedwhich effectively results in a logical AND operation of the criteria.Multiple exceptions 612 can be selected which effectively results in alogical OR operation of the exceptions; that is, any one of theexception conditions being true will result in a policy not beingtriggered. In another embodiment, a generic Boolean expression is usedin lieu of the criteria and exception combination. Annotation actions614 cause generation of attachment to message 602 or insertion of textinto the body 208 of the message. The manner by which annotations aremade is based on a policy entered by the administrator. Notificationactions 616 cause the sending of one or more e-mail notifications when agiven policy is triggered. Notifications can be sent to sender,recipient, administrator, or any e-mail address that is defined by theadministrator. In addition, notification actions 616 allow specificationof whether the original message 204 should accompany the notification.Disposition action 620 determines whether the message should continue tothe destination(s) (specified by field 620) or whether one of aplurality of alternative actions 622 such as deferral, quarantine,return to sender, or dropping of the message are required.

In one embodiment, the policy action dictates that a digital signatureshould be detected and verified in accordance with signature attributes.FIG. 8 illustrates the operation of a signature verification portion ofthe security manager 226. In one embodiment, the security manager 226executes a signature verification policy that verifies electronicsignatures in received e-mail messages. The verification processpreferably starts with identifying that the received e-mail messageincludes an electronic signature (step 801). In one embodiment, thesecurity manager 226 applies a policy to determine whether the e-mailmessage is such that the signature should be verified. In otherembodiments, the security manager 226 automatically initiates thesignature verification process in response to detecting a signature,regardless of the e-mail message attributes. The security manager 226applies a security policy for the signature (step 803). The securitypolicy preferably specifies the level of verification that is required,based on e-mail message attributes. Once the signature policy is appliedto the received e-mail message, the security manager proceeds to verifythe signature in accordance with the policy.

The security manager 226 starts by hashing the signed e-mail message toextract a message digest. The signature is then decrypted using thesigner's public key, to produce a second message digest, the originalmessage digest. The two message digests are compared to verify that theyare identical. The security manager 226 proceeds to verify that thepublic key used in the verification belong to the sending entity. Suchverification is accomplished by verifying the signer's digitalcertificate, which is included with the signature. As discussed above,the verification level is preferably determined by the security policythat is applicable to the current e-mail message. The policy actionsinclude verifying the signing certificate against a configurable list,verifying the digital certificate validity dates, verifying the keystrength and algorithm allowed by the certificate, verifying thecertificate usage (i.e., can the certificate be used for signing),verifying the certificate chain, verifying that the root certificate isin a list of acceptable root certificates, and verifying that thecertificate is not revoked.

In one embodiment, the digital certificate verification is simplified byquerying a local directory of acceptable signing digital certificates,followed by the querying of one or more trusted remote servers. Thesecurity manager 226 searches for the digital certificate in a localdirectory, which stores trusted digital certificates that do not requirefull verification (step 805). If the digital certificate is located inthe local trusted digital certificate directory, the signatureverification process is reported as successful (step 807). If thedigital certificate is not in the trusted digital certificate directory,the server proceeds to search for the digital certificate in one or moretrusted remote directories (step 809). If the security manager 226receives the digital certificate from one of the trusted remotedirectories, the signature is deemed valid (step 810). The securitymanager 226 provides a corresponding result notification to the policymanager so as to facilitate proper follow up actions, such as rejectionor acceptance of the e-mail message. In one embodiment, the notificationis in the form of a text message that is appended to a received message.

In another embodiment, one or more trusted signature verificationservers are used to verify signatures so as to provide for theoff-loading of complex signature verification operations from the e-mailfirewall. One may appreciate that the digital signature verificationoperation consumes substantial processing power of the e-mail firewall,as well as adding administrative burden, because of the need to maintainroot certificates, intermediate certificates, acceptable signingcertificates, and certificate revocation lists (CRLs). Accordingly, inthis embodiment, the security manager hashes the e-mail message andsubmits the resultant data to the signature verification server forperforming the verification externally. In one embodiment, the dataincludes the computed hash, the signature information (including thehash as encrypted by the sender and signing digital certificate), andpolicy data, which indicates the required verification level. Thesignature verification server receives the data from the securitymanager and processes the data substantially as the local securitymanager does in the previously discussed embodiment. Such processingincludes verifying certificate validity dates, certificate usage,certificate chain, certificate non-revocation, and root certificate.After determining whether the signature is valid, the verificationserver transmits a corresponding response to the security manager 226 ofthe e-mail firewall. The e-mail firewall proceeds in accordance withactions, as dictated by the applicable policy and verification results.

In one embodiment, the signature verification server is a trusted serverand the communication between the e-mail firewall and the signatureverification server is authenticated. In this embodiment, the secureconnection is facilitated by an SSL connection or by requiring thesigning verification server to sign the response. Although thisauthentication method requires the e-mail firewall to verify asignature, such verification does not draw much processing power sincethe e-mail firewall employs a known set of signature verificationservers and accordingly can locally store the verification certificates.Preferably, the communication protocols used by the e-mail firewall andthe signature verification server include XML, ASN.1 encoding, and MIME.

Referring now back to FIG. 6( b), the illustrated steps are performedfor each destination specified for a message 204. The steps shown inFIG. 6( b) are also performed for messages generated by step 622. First,policy managers 216 perform actions 610, 612, 614 and 616, for eachdestination specified in the message 204. Disposition action 623,operates similarly to disposition action 620 by determining whether themessage should continue to the destination(s) or whether one of aplurality of alternative actions 622 such as deferral, quarantine,return to sender, or dropping of the message, are required. At step 624,a determination is made if encryption or signature is required. Ifencryption is required, then at step 626 encryption is performed inaccordance with stored keys 628. If a signature is required, a signatureis added at step 629. Notice that some implementation may instead chooseto sign before encrypting. The message is then transmitted to thespecified destination at step 630. Messages that are processed by block622 are also checked at step 624 before transmission. For example,messages that are deferred, quarantined, or returned to the sender, mayneed to be encrypted or include a signature.

FIG. 9 illustrates a signing operation, which is preformed by thesecurity manager 226 when processing an e-mail message for transmission.The process illustrated in FIG. 9 is for adding a signature to an e-mailmessage in accordance with policy determinations as applicable to step629 in FIG. 6( b). Applying signatures to e-mail messages is well knownin the art, as is discussed in the S/MIME Standard. The e-mail firewallhas available a signature inclusion policy for defining the e-mailmessages to which a signature is added (step 901). The e-mail firewalldetermines if the e-mail message is such that a signature is added. Inone embodiment, an e-mail firewall policy refers to the e-mail messagetextual content, destination, source, and size, in determining whether asignature is required. If a signature is required for the e-mailmessage, the security manager 226 applies a signature selection policyso as to identify a corresponding signature for the e-mail message (step903). The security manager 226 retrieves a signature in accordance withthe signature selection policy (step 905). The signature is applied tothe e-mail message (step 904). The e-mail message is then preferablyforwarded to the policy managers for further processing (step 909).

When encryption is required, the security manager 226 retrievescorresponding public keys for the e-mail recipient. FIG. 10 is a flowdiagram illustrating the operation of a certificate lookup module (notshown) of the security manager 226. As is discussed above, certificatesare employed by the security manager 226 to securely transmit e-mailmessages. In one embodiment, the policy dictates encryption for one ormore recipients in accordance with the method that was discussed withreference to FIG. 6( a). In another embodiment, the policy dictatesencryption for an e-mail firewall of one or more recipients inaccordance with the method that was discussed with reference to FIG. 6(b). In both methods, the security manager 226 is accessing therecipient's, or the e-mail firewall's, public key. Hence, the securitymanager 226 retrieves digital certificates, which are typically used tostore public keys. When the source of digital certificates is not fullytrusted, the security manager 226 first verifies the validity of thedigital certificate before employing it to encrypt a message. Theverification of encryption certificates is preferably in accordance withthe process discussed above with reference to the signature certificateverification process.

In one embodiment, the security manager includes a local persistentmapping from recipient e-mails and/or SMTP server domain to digitalcertificate. This mapping is referred to as a local digital certificatedatabase. The local digital certificate database is configured andmaintained by the system administrator of the e-mail firewall. The localdatabase is usually considered trusted by the security manager, therebyoptionally relieving the requirement for verification of digitalcertificate validity. The maintenance and query of such a database iswell known in the art, such as by employing a key-value database or arelational database.

In another embodiment, the security manager 226 uses one or more remotecertificate lookup servers in addition to, or in lieu of, the localdigital certificate database of the previous embodiment. In thisembodiment, the maintenance of the remote certificate database is notperformed by the system administrator of the e-mail firewall but is bythe system administrators of remote certificate lookup servers, whichcan be controlled by a trusted third party, such as a CertificateAuthority (CA). FIG. 12 illustrates such arrangement where an externalcertificate lookup server 1210 is employed to provide security data,including certificate data, to e-mail firewalls 1202, 1203. A firstplurality of user computers 1208 is coupled to a first firewall 1202 bya local network connection. In one embodiment, the local networkconnection between the user computers 1208 and the e-mail firewall 1202is a secure private network, as is known in the art. The first e-mailfirewall is coupled to a public network 1204, such as the internet, by anetwork connection. A second e-mail firewall 1203 is also coupled to thepublic network 1204 by a network connection. A second plurality of usercomputers 1206 is coupled to the second e-mail firewall 1203. The secondplurality of user computers 1206 is preferably also coupled to theassociated e-mail firewall 1203 by a secure private network. In anotherembodiment, the user computers are provided by the combination of userterminals and a corresponding mainframe server, as is known in the art.

A certificate lookup server 1210 is coupled to the public network 1204by a network connection. The certificate lookup server 1210 preferablystores security data that is available to security processes in thefirewalls 1202, 1203 for facilitating secure communication of e-mailmessages over the public network 1204.

In another embodiment, the certificate lookup server is replaced by oneor more intermediate e-mail firewalls which act as intermediate e-mailrelays between the first e-mail firewall 1202 and the second e-mailfirewall 1203. The ability to use an intermediate e-mail relay in astore-and-forward protocol such as SMTP is well known in the art. Anintermediate e-mail firewall preferably receives an e-mail message froma sending firewall, encrypted for the subject intermediate e-mailfirewall. The intermediate e-mail firewall then decrypts the subjecte-mail, using its private certificate, and re-encrypts the subjecte-mail, using the recipient e-mail firewall's public certificate.Finally, the intermediate e-mail firewall forwards the e-mail message tothe recipient's firewall. Accordingly, the intermediate e-mail firewallis the only entity that needs access to the security data forrecipient's e-mail firewalls. In this embodiment, the sending e-mailfirewall locally stores encryption certificates for the intermediatee-mail firewall it is using to transmit secure e-mail messages withoutaccessing any certificate retrieval directory. Accordingly, regardlessof the intended recipient, the sending e-mail firewall employs the sameencryption certificate to transmit encrypted e-mail messages to therecipient by way of the intermediate e-mail firewall. There is no needto employ multiple certificates for different recipient e-mailfirewalls. Furthermore, there is no need to retrieve such certificatesfrom the external certificate lookup server.

Turning back to FIG. 10, the certificate lookup module optionally startsby a local search, as in the first embodiment, and resets the availablecertificates list so as to contain locally stored trusted certificatesmatching the recipient's e-mail or the recipient's server domain, if anyone available (step 1001). The certificate lookup module then submits aremote query to one of several privately or publicly availablecertificate lookup servers, requesting certificates associated with therecipient e-mail or with the recipient's server. The certificate lookupmodule preferably employs the Lightweight Directory Access Protocol(LDAP) to query the remote servers for certificates. However, use ofother standard or non-standard lookup protocols is not precluded. If thecertificate server responds with certificates, the certificates areadded to the available certificates list (step 1005). If the certificatelookup server returns no certificates, the certificate lookup moduleproceeds to the next certificate lookup server, if any such serversremain (step 1007). After adding certificates to the availablecertificate list in step 1005, the certificate lookup module proceeds tovalidate the certificates in the list (steps 1009, 1011, 1013).Alternatively, to offer the largest choice of certificates to the policyengine, the lookup is performed against all servers even after somecertificates are found. Furthermore, in other embodiments, instead ofquerying the certificate lookup servers in sequence, remote servers arequeried in parallel using multithreading programming techniques, whichare well-known in the art. Next, the certificate lookup module proceedsto apply verification policy to the certificates in the availablecertificates list (steps 1009, 1011, 1013).

For each certificate in the list, policy is applied to determine whetherthe certificate is acceptable (step 1011). In one embodiment, suchpolicies refer to the certificate's validity dates, usage restrictions,chain verification, root verification, key strength, algorithmrestrictions, and presence in one or more Certificate Revocation Lists(CRLs). A certificate that is deemed invalid is discarded from theacceptable certificate list.

Once the list of acceptable certificates is finalized, the policyoptionally specifies preferences used to sort the list of acceptablecertificates. In one embodiment, a policy assigns preference tocertificates with longer keys or to certificates issued by one CA overanother CA. The policy preferably dictates to use the top certificate ofthe list. In another embodiment, the policy dictates encrypting to thetop 2, 3, or N certificates. If none of the certificates are acceptable,a corresponding message is provided to the security manager 226 toindicate that encryption is not available for the recipient. In oneembodiment, the resulting certificates are stored in a local flatdatabase, as is known in the art, which can act as a local cache of theremote certificate lookup servers.

In another embodiment, the policy requires the certificates to beverified against a Certificate Revocation List (CRL). The certificatelookup module accesses a CRL on one or more remote server systems, whichcan be different from the certificate lookup servers. Preferably, theCRL is published in predetermined locations that are accessible to thelookup module. In one embodiment, the lookup module employs the OnlineCertificate Status Protocol (OCSP), which defines a protocol forsubmitting a certificate identifier and receiving a response regardingthe revocation status of the certificate.

In another embodiment, the complex task of searching for and validatingcertificates, as well as applying the policy requirements andpreferences to the matching certificates (FIG. 10), is delegated to anexternal trusted server (herein certificate lookup and verificationserver). The advantages of this embodiment parallel those of thesignature verification server, described earlier in this application,which include removing complexity from the e-mail firewall server andsimplifying system administration by delegating tasks to a remoteserver, administered by a trusted third party, such as a CA. In thisembodiment, the e-mail firewall submits the e-mail address of therecipient, or the domain of the recipient's e-mail, to the certificatelookup and verification server and optionally submitting a descriptionof the policy requirements, or preferences, for the certificates. Thecertificate lookup and verification server responds by facilitating thelookup and verification according to its own policies or according tothe policies submitted by the e-mail firewall. The processing logic ofthe certificate lookup and verification server is similar to the logicof the security manager 226 as discussed with reference to the previousembodiment. Such logic includes querying local storage of the lookup andverification server, querying remote certificate lookup servers,verifying certificates according to policy requirements, and sortingcertificates according to policy preferences. The response, whichincludes a sorted list of one or more certificates, is returned by thecertificate lookup and verification server to the security manager 226of the e-mail firewall. The e-mail firewall can then use thesecertificates to perform encryption.

As may be appreciated, the certificate lookup and verification server ispreferably a trusted server. Furthermore, the communication between thee-mail firewall and the certificate lookup and verification server ispreferably authenticated. The authentication can be achieved with SSLconnection or by having the certificate lookup and verification serversign its answer. As discussed above, although this authentication methodrequires the e-mail firewall to verify a signature, such verificationdoes not draw much processing power since the e-mail firewall employs aknown set of certificate lookup and verification servers and accordinglycan locally store these certificates. The communication encoding used bythe e-mail firewall and the certificate lookup and verification serverinclude XML, ASN.1 encoding, and MIME. It should also be appreciatedthat the certificate lookup and verification server can be combined withthe signature verification server described earlier. It should also beappreciated that for network performance the e-mail firewall may submitseveral request to the certificate lookup and verification server in onenetwork transaction instead of performing a transaction for eachrecipient. The transport of requests and responses between the e-mailfirewall and the certificate lookup and verification server includeplain TCP/IP socket, HTTP transaction, secure HTTP transaction, CommonObject Request Broker (CORBA) invocation, Remote Method Invocation(RMI), and Remote Procedure Call (RPC) invocation.

FIG. 11 illustrates further details of the operation of the securitymanager during the encryption step 626 of FIG. 6( b). It should beappreciated that whatever lookup method is used to retrieve the digitalcertificate during the encryption process, the available certificatescan include more than one certificate. In one embodiment, policies areemployed to enforce requirements on the encryption certificates, such asminimum key length or specifying a root Certificate Authority (CA).These requirements, provided by applicable policies for the processedmessage, are then used to filter the available certificates, down to afinal list of acceptable encryption certificates. The security manager226 retrieves available certificates for encrypting e-mail messages(Step 1103). In one embodiment, the certificates are retrieved from alocal source. In another embodiment, the security manager 226 employs alookup module to submit a query to a public security server and retrievecertificates for the desired server. The security manager 226 continuesto determine whether any certificates were retrieved, regardless ofsource. If no certificates were retrieved, the security manager 226indicates that encryption has failed for the intended recipient (Step1105). In one embodiment, if certificates are retrieved, the securitymanager 226 proceeds to validate the certificates by employing anexternal directory, as discussed above (step 1107). In yet anotherembodiment, the security manager 226 proceeds without validating thecertificate. Preferably, when an acceptable certificate list isemployed, as discussed with reference to FIG. 10, the security manager226 does not validate the certificates. The preferences specified by theapplicable policies are then used to generate a list of acceptableencryption certificates in order of decreasing preference (Step 1110).The top certificate in the sorted list is preferably used forencryption. In one embodiment, this policy filtering procedure ispreformed by the security manager 226 on a per message basis. In otherembodiments, the policy filtering procedure is by the security manager226 on a firewall by firewall basis, as configured by an administrator.The security manager 226 proceeds to encrypt the e-mail message byemploying the retrieved certificate (step 1109). After encryption of themessage the security manager 226 continues processing the message, asdiscussed with reference to FIG. 6( b).

FIG. 7 is a block diagram showing further details of alternative actions622. Messages received from disposition step 620 are stored in one ofthe four queues 702, which include quarantine queue 704, retry queue706, dead letter queue 708, and defer queue 709 depending upon thespecified disposition of the message. Quarantine queue 704 storesmessages for subsequent retrieval and review by a system administratoror other authorized person. Retry queue 706 stores messages for whichdelivery has failed. Transmission of messages in the retry queue 706 issubsequently re-attempted. Dead letter queue 708 stores messages whichcontinue to be undeliverable after several retries and which cannot bereturned to the sender. Messages in the dead letter queue 708 may beacted upon by a system administrator. Defer queue 709 stores messages tobe delivered automatically at a later time, for example an off-peak-timesuch as a weekend or night time. Configuration module 230 provides aplurality of actions 710-714 which may be performed on the messages inqueue 702. The messages can be viewed 710 by the administrator, returnedto the sender 711, deleted 712, sent to the specified destination(s) 713and/or saved 714.

Although the present invention was discussed in terms of certainpreferred embodiments, the invention is not limited to such embodiments.Rather, the invention includes other embodiments including thoseapparent to a person of ordinary skill in the art. Thus, the scope ofthe invention should not be limited by the preceding description butshould be ascertained by reference to the claims that follow.

1. A method for signing an e-mail message, the method comprising:receiving at an e-mail firewall, a plurality of e-mail messagesincluding at least first and second messages from different usersassociated with different user systems; with a security manager of thee-mail firewall, determining for each of the received first and secondmessages if a signature is required by applying a signature policy thatreferences particular attributes of the first and second messages,respectively; retrieving a signing certificate in accordance with thesignature policy for application to at least the first message; andforwarding both the first and second messages for further processing bythe e-mail firewall.
 2. The method of claim 1 wherein said determiningis by reference to content analysis of an e-mail message for sensitivityclassification.
 3. The method of claim 1 wherein said application of thesigning certificate includes signing by employing a key-based signingscheme which does not include the use of an X.509 certificate.
 4. Themethod of claim 3 wherein said application of the signing certificateincludes PGP signing.
 5. The method of claim 3 wherein said applicationof the signing certificate includes RSA signing.
 6. The method of claim1 wherein said retrieving is by reference to attributes of an e-mailmessage and is facilitated by interaction with an external identitymanagement service.
 7. The method of claim 6 wherein said externalidentity management service is an LDAP service.
 8. The method of claim 6wherein said external identity management service is an ActiveDirectoryserver.
 9. The method of claim 1, wherein the attributes referenced bythe signature policy include one or more of content, destination, sourceand size of the messages.
 10. The method of claim 1, further comprising:selecting and applying a respective signing certificate to the secondmessage.
 11. The method of claim 1, further comprising: determining,based on the signature policy, that no signing certificate is to beapplied to the second message.
 12. The method of claim 1, furthercomprising: based on a signature selection policy and the referencedattributes, selecting and applying a same signing certificate to boththe first and second messages.
 13. The method of claim 1, furthercomprising: based on a signature selection policy and the referencedattributes, selecting and applying different signing certificates to thefirst and second messages.
 14. A computer program product stored in oneor more computer readable media, the program product comprisinginstruction sequences executable on a computer to: implement a securitymanager of a messaging firewall that determines, for individual receivedmessages, if a signature is required and that applies a signature policythat references particular attributes of the individual receivedmessages in the determination; retrieve a first signing certificate,apply the first signing certificate and thereby sign at least a firstone of the received messages based on the signature policy; and forwardboth the signed first message and a second one of the received messagesfor further processing by the messaging firewall.
 15. The computerprogram product of claim 14, wherein the particular attributesreferenced by the signature policy include one or more of content,destination, source and size of the individual received messages. 16.The computer program product of claim 14, wherein the instructionsequences are further executable to: retrieve a second signingcertificate and to apply the second signing certificate to at least thesecond message based on the signature policy and based on particularattributes of the second message.
 17. The computer program product ofclaim 14, wherein the instruction sequences are further executable to:apply the first signing certificate to both the first and secondmessages based on the signature policy and based on particularattributes of the first and second messages.
 18. The computer programproduct of claim 14, wherein the messages include e-mail messages, andwherein the messaging firewall is configured to be interposed in SMTPprotocol traffic flows.
 19. The computer program product of claim 14,further comprising: an interface to an external identity managementservice from which the first signing certificate is retrievable.
 20. Amessaging firewall comprising: a message relay for receiving a pluralityof electronic messages from different users associated with differentuser systems; a security manager that determines, for each of thereceived electronic messages, if inclusion of a signature is required byapplying a signature policy that references particular attributes of thereceived electronic messages; a signing certificate retrieval interfaceresponsive to the security manager, the signing certificate retrievalinterface retrieving signing certificates selected for application torespective ones of the received electronic messages in accordance withthe signature inclusion determinations of the policy manager; and amessage transmission interface for transmitting both signed and unsignedones of the received electronic messages in accordance with thesignature inclusion determinations of the policy manager.
 21. Themessaging firewall of claim 20, wherein the particular attributesreferenced by the signature policy include one or more of content,destination, source and size of the individual received messages. 22.The messaging firewall of claim 20, wherein the signing certificateretrieval interface retrieves a second signing certificate and themessaging firewall applies the second signing certificate to at leastthe second message based on the signature policy and based on particularattributes of the second message.
 23. The messaging firewall of claim20, wherein the first signing certificate is applied by the messagingfirewall to both the first and second messages based on the signaturepolicy and based on particular attributes of the first and secondmessages.
 24. The messaging firewall of claim 20, wherein the messagesinclude e-mail messages, and wherein the messaging firewall isconfigured to be interposed in SMTP protocol traffic flows.
 25. Themessaging firewall of claim 20, further comprising: an interface to anexternal identity management service from which at least the firstsigning certificate is retrievable.
 26. A method for verifying a digitalsignature in a received message, the method comprising: receiving at amessaging firewall, a plurality of electronic messages including atleast first and second messages from different users associated withdifferent user systems; with a security manager of the messagingfirewall, determining for each of the received first and second messagesa level of verification required by applying a signature verificationpolicy that references particular attributes of the first and secondmessages; attempting to verify the first and second messages todiffering levels of confidence in accordance with signature verificationpolicy and the particular attributes of the respective message.